Shield sheets for preventing the leak and intrusion of electromagnetic waves are used in electronic and communications apparatuses and systems such as personal computers, cell phones, electronic toll-collecting (ETC) systems on toll roads, wireless LAN, etc. The shield sheets are required to efficiently absorb electromagnetic waves in a wide frequency range, with little absorbability difference (anisotropy) depending on incident directions. Particularly, in systems using circularly polarized electromagnetic waves such as ETC, etc., shield sheets are required to efficiently absorb both of TE waves (electromagnetic waves whose electric field components are perpendicular to an incident surface) and TM waves (electromagnetic waves whose magnetic field components are perpendicular to an incident surface).
Metal sheets or nets now widely used as shield sheets are heavy, and need much time to be arranged in apparatus casings. In addition, metal sheets or nets have large anisotropy in electromagnetic wave absorbability, resulting in drastic decrease in electromagnetic wave absorbability as the incident angles of electromagnetic waves increase.
As a light-weight, electromagnetic-wave-absorbing shield sheet easy to be arranged in a casing, JP 9-148782 A proposes a shield sheet comprising a plastic film, and first and second vapor-deposited aluminum layers formed on both surfaces thereof, the first vapor-deposited aluminum layer being etched to a non-conductive linear pattern, and the second vapor-deposited aluminum layer being etched to a conductive network pattern. However, because this shield sheet has a periodic linear pattern and a periodic network pattern, it cannot efficiently absorb electromagnetic waves in a wide frequency range, and its electromagnetic wave absorbability has large anisotropy.
JP 2004-134528 A proposes an electromagnetic wave absorber comprising an anisotropic resistance layer, a dielectric layer containing conductive fillers, and an electromagnetic-wave-reflecting layer in this order, the anisotropic resistance layer comprising conductive line patterns, and having anisotropy with surface resistance of 1 kΩ or less in one direction and 10 kΩ or more in other directions. JP 2004-134528 A describes that the arrangement of the electromagnetic wave absorber with the line patterns in parallel to the magnetic field components of TE waves can absorb both TE waves and TM waves efficiently. However, this electromagnetic wave absorber suffers large anisotropy in electromagnetic wave absorbability.